Turntable speed accuracy

H Lewm, yes, you are correct. That is called cogging. I'm sure BD motors do it too if they are unloaded. You just can't see it with your eyes because they spin too fast. That is what I was saying before that designers use mass as a damper- mass damping. All motors have some level of cogging, or ripple. Even your car engine has it as each piston fires. The flywheel smoothes out those ripples just as the tt platter smoothes out the ripples from the motor. The typical tt specs showing Wow&Flutter below 0.03% shows how well these massive platters are doing their job. btw, the more poles that a motor has, the smaller the ripples. I notice some of these expensive tables have quite sophisticated motors. Caveat Emptor always applies, but I think you are getting what you pay for in some of these expensive motor designs.

Tony you may find Dovers record player interesting ,surely a heavy weight contender for ultra sophisticated speed control.

Henry if my memory is correct I seam to think that I read somewhere TW Acustics stated their motor speed control was designed to perform like a vintage direct drive.

Dear Tony, Cogging is indeed a problem with motors, but what I described would not be strictly due to cogging. Mostly it is due to the servo hunting when its feedback loop is screwed up by the lack of a platter mass or in the other extreme, too much platter mass. Cogging will always be there and is an inherent property of the motor; some are less prone to cogging than others.

In a way we should think different, because speed stability is one side, sound quality has absolutely nothing to do with it and a turntable which creates "something" is also a total different animal (Raul is absolutely right on that topic).
When you want speed stability, a direct drive has advantages because it always is in connection with the used voltage, it always correct its speed during play (accelerate-hold-reduce-accelerate-hold-reduce and so on).
Direct drives are good for those who believe, that this has something to do with superior sound quality. A listened to some really good designs and I know, this has absolutely no influence. A direct drive TT can sound thin, lifeless and boring. Maybe there are exceptions ...
A belt drive TT Designer has to solve some problems, see Belt quality for example. Most aren't even able to solve this properly, then we can't expect that those are able to design a proper speed control. and this story goes on and on.
When someone is reading this and has no time to read the former 3 sites
- go for any suspended Basis turntable with Basis Controller or Walker Controller
or
- ask Sorasound for an Amazon turntable, they have a Battery power supply.
That one is good.
- or SME 20/30

Others produce "something" what someone likes or not. Discussions about "this" are endless because one prefers black, white, heavy, expensive, rare, PRaT, catholic or islamic soundstage and so on. These "problems" were all solved years ago, unfortunately engineering is no longer part of High End Analog. It is replaced with "I like it". Analog is a following of steps done right.

Gosh, what is left to say or think after that?
I agree that direct-drive turntables can be highly "colored", in a bad way. I also completely disagree; speed stability is the sine qua non of a turntable, no matter the drive system. "Sound quality", if by that term you mean the degree to which the sound emanating from the speakers can be made to emulate "real life" does depend very much upon speed stability. From that comes rhythm. From rhythm comes verisimilitude, in part.

Sure, a tt with excellent speed stability can sound bad for other reasons.

Why is a battery power supply, per se, likely to be superior to all other approaches to power supply?